1. Filed of the Invention
The present invention relates to a thin film forming device for forming a thin film on a base material by the plasma CVD (Chemical Vapor Deposition) method. More particularly, the present invention relates to the device capable of forming an excellently crystalline thin film without heating a base material to high temperature.
2. Description of the Related Art
In order to manufacture a thin film transistor (TFT) for composing a liquid crystal display, a semiconductor integrated circuit or a solar battery, a thin crystalline film is formed on a base material, for example, a thin silicon film is formed on the base material.
One of the means for forming the thin crystalline film is the plasma CVD method. According to the plasma CVD method, raw material gas is decomposed in plasma so that a thin film is formed on the base material. Therefore, the plasma CVD method is advantageous in that the base material can be kept at a relatively low temperature.
FIG. 5 is a view showing a conventional example of a thin film forming device to which the plasma CVD method is applied. In a reaction chamber container 2, from which gas is exhausted to vacuum by a vacuum exhauster not shown in the drawing, there are provided a base material holder 8 for holding a base material 10, and a high frequency electrode 12 which are opposed to each other. In the base material holder 8, a heater 9 is provided for heating the base material. Raw material gas 6 such as silane (SiH.sub.4 /H.sub.1) diluted by hydrogen is introduced into the reaction chamber container 2 via a gas introduction pipe 4. Electric power of high frequency is supplied between the high frequency electrode 12 and the base material holder 8 from a high frequency electric power source 16 via a matching circuit 18.
When the raw material gas 6 is introduced and the electric power of high frequency is supplied as described above, electric discharge of high frequency is generated between the high frequency electrode 12 and the base material holder 8. By this electric discharge of high frequency, the raw material gas 6 is ionized and plasma 20 is generated. The raw material gas 6 is decomposed in this plasma 20, and a thin film is formed on the base material 10, for example, a thin film of silicon is formed on the base material 10.
In the above thin film formation device, the plasma CVD method is used. Therefore, it is possible to keep the base material at a relatively low temperature. However, in order to form a thin crystalline film, it is necessary to heat the base material 10 to at least 900.degree. C. by the heater 9. Therefore, the base material 10 must withstand this temperature. For the above reasons, a type of the base material 10 is limited. For example, glass of a low softening point such as soda glass or non-alkali glass, the softening point of the former is approximately 400.degree. C. and the softening point of the latter is approximately 700.degree. C., is not expensive. However, it is impossible to use the above glass of a low softening point in the conventional device.
Further, since the base material 10 is directly exposed to the plasma 20, ions existing in the plasma 20 having energy of various intensities are incident on a surface of the base material 10. The ions having energy of various intensities are incident on the base material 10, and the ions are always incident in the process of film formation. Consequently, an excessively large quantity of ions are incident on the base material 10. Therefore, growth of crystals of the thin film formed on the base material is obstructed, and damage (defect) is caused on the film, that is, it is difficult to form an excellently crystalline thin film.